In the manufacture of photographic film and paper, the liquefaction of gelled materials is required prior to coating these materials on the film or paper. Gelled photographic materials include aqueous or solvent based photosensitive or non-photosensitive emulsions and dispersions.
There are two general methods that are known for liquefying gelled photographic materials: batch liquefaction; and continuous liquefaction. Batch liquefaction utilizes a jacketed kettle fitted with a stirring device. Heat is provided to the vessel through hot water or steam. The jacket system also provides chilled water for cooling. The jacket generally utilizes spray rings and operates under thin film heat transfer conditions. Jackets may also operate under flooded conditions although it is less common.
Continuous liquefaction may be dynamic or passive. In dynamic continuous liquefaction, gelled material is forced into a heat exchanger by means of an auger and/or positive displacement pump. The system may include a vacuum chamber to remove air from the solid gelled material. Material is liquefied in the heat exchanger and conveyed to the coating station. Examples of continuous liquefaction devices are found in U.S. Pat. No. 5,182,190, PCT Publication WO 89/06829 and PCT Publication WO 92/09007.
Dynamic continuous liquefaction equipment may damage or alter photographic materials due to mechanical energy from the pump or auger. Additionally, the close tolerance pumps typically used in dynamic liquefaction applications are subject to damage from foreign materials which may be present in the photographic materials, and will reduce overall machine reliability.
Passive continuous liquefaction typically uses a flat coil or heating grid to liquefy material at the required coating rate. Liquefied gelatin passes through gaps between the coil or grid and is collected in a low volume reservoir. The reservoir may include an ultrasonic horn to enhance debubbling. The liquid level in the reservoir is monitored and used to control the heating rate of the coil or grid to maintain a constant rate of liquefaction. An example of such a passive liquefaction device can be found in U.S. patent application Ser. No. 07/815,462, now abandoned.
There exists a need to use both conventional and passive liquefaction technology in the same vessels so as to accommodate a wide range of product and production conditions. The coil type technology does not readily meet those needs. There also are concerns around the ability to rapidly clean the coil to accommodate rapid changes in product type. The present invention provides the advantages of passive liquefaction while utilizing existing vessels in a manner which accommodates both rapid product changes and allows changes between continuous and batch type operations.